1. Field of the Invention
The present invention relates to a method and system of filtering signal data. More specifically, the present invention relates to a method and system for analyzing signal data collected by an array of receivers, determining the coherence function of the received data, and filtering the received raw data with the coherence function.
2. Description of Related Art
Data collection arrays, i.e. a collection of more than one single position point data recorders, are used in the collection of a myriad of data. Examples of array collected data include radar, seismic, acoustic, sonar, radio waves, to name but a few. Often the data received and recorded by such arrays can include unwanted signals that intermingle with the desired data and distort the final recordings thereby providing skewed results. Moreover, when dealing with arrays of data recording devices, the time lag between signals of the individual recorders is especially important. While the recorded data can be processed and filtered to remove the noise and to extract information from the time lag, there still exists room for significant improvement in processing such data.
In acoustic logging through an earth formation, acoustic signals that travel along the formation are often contaminated by other acoustic waves that travel along a different path. For example, in the logging-while-drilling measurement, acoustic waves may travel long the tool body (drill collar) and significantly interfere with the formation signals. In cased hole logging, acoustic waves transmission along the casing may become significant if the casing is poorly bonded with cement. Moreover, this wave transmission may become overwhelmingly large if the casing is detached from cement (i.e., the free-pipe situation). Tubman, K. M., Cheng, C. H., and Toksoz, M. N., 1984, Synthetic full-waveform acoustic logs in cased borehols, II—Poorly Bonded Casing, Geophysics, 51, 902,913. In the latter situation, processing acoustic signals to obtain formation properties is extremely difficult because the formation signals are almost in discernible in the presence of overwhelming casing waves. Because the majority of existing wells are cased and there is often a need to determine acoustic properties through casing, acoustic logging in cased borehole is still routinely made because formation properties can be measured when casing is well bonded with formation. Tang, X. M., and Cheng, C. H., 2004, Quantitative Borehole Acoustic Methods, Elesevier. However, the measured data are often abandoned when casing is poorly bonded or detached because of the interference caused by the waves passing through the casing.
Because of the need to measure formation acoustic properties through a poorly bonded/detached casing, various methods have been tested to process acoustic data under these conditions. A common practice is to apply the routine semblance method directly to the data and to detect the small events on the semblance correlogram that are associated with the formation arrivals. This method often fails because the formation signals, although they theoretically exist, are small compared to the large casing ringing signals and are thus difficult to distinguish from noises in the data. A maximum likelihood method was also used to enhance the resolution of the formation signal on the correlogram. Block, L. V., Cheng, C. H. and Duckworth, G. L., 1986, Velocity Analysis of Multi-receiver Full Waveform Acoustic Logging Data in Open and Cased Holes, 56th Ann. Internat. Mtg.: Soc. of Expl. Geophys., Session:BHG2.5. However, due to the amplitude difference between the formation and casing signals, the enhanced resolution is not of much help in resolving the low-coherence formation signal with a poor signal-to-noise ratio.
A need exists to suppress the strong casing signal so that the formation signal coherence can be enhanced, to accomplish this a waveform subtraction method was developed to suppress the casing signals. Valero, H., Skelton, O., Almeida, M., Stammeijer, J. and Omerod, M., 2003, Processing of Monopole Sonic Waveforms Through Cased Hole, 73rd Ann. Internat. Mtg.: Soc. of Expl. Geophys., 285-288. By insolating a portion of the casing waves ahead of the formation arrival and subtracting the waves from the data, the formation signal coherence is enhanced and the signal can thus be picked from the semblance processing. However, as stated in Varelo et al (2003), the method does not work well when the casing and formation signals overlap in time. Therefore, there exists a need for a device and method capable of processing signal data and successfully filtering away unwanted portions of the acquired signal.